Most commonly using a soothing female voice, Siri answers questions, fulfills tasks and manages users' calendars. But it comes in a number of voice options — male or female, with accents including American, British, and Australian — and these are based on human voice actors.

In selecting these actors, "first and foremost, a voice must be perceived as being compatible with the Siri personality," Apple engineers wrote.

They don't elaborate on exactly what the "Siri personality" is — but it typically comes across as restrained, neutral and professional, with the occasionally dryly delivered joke for those who know what to ask.

Once a suitable voice talent is found, between 10 and 20 hours is recorded of their voice. "The recording scripts vary from audio books to navigation instructions, and from prompted answers to witty jokes," Apple's Siri team wrote in a blog post.

"Typically, this natural speech cannot be used as it is recorded because it is impossible to record all possible utterances the assistant may speak." As such, it's then chopped up into constituent blocks that can be put together using to generate new speech — even words that the actors never uttered.

The tricky part is constructing Siri's speech in a way that sounds natural and "human" — and to do this, Apple uses a number of artificial intelligence (AI) techniques. The company's researchers go into more technical detail on how they achieve this in newly published papers.